β-allyl sulfones as addition-fragmentation chain transfer reagents: A tool for adjusting thermal and mechanical properties of dimethacrylate networks

Article abstract of DOI:10.1021/ma501550b

Dimethacrylates are known to have good photoreactivity, but their radical polymerization usually leads to irregular, highly cross-linked, and brittle polymer networks with broad thermal polymer phase transitions. Here, the synthesis of mono- and difunctio

Full text of DOI:10.1021/ma501550b

Article
pubs.acs.org/Macromolecules
β‑Allyl Sulfones as Addition−Fragmentation Chain Transfer
Reagents: A Tool for Adjusting Thermal and Mechanical Properties of
Dimethacrylate Networks
Christian Gorsche,†,‡ Markus Griesser,†,‡ Georg Gescheidt,§ Norbert Moszner,‡,∥ and Robert Liska*,†,‡
†Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163 MC, 1060 Vienna, Austria
‡Christian-Doppler-Laboratory for Photopolymers in Digital and Restorative Dentistry, Getreidemarkt 9, 1060 Vienna, Austria
§Institute of Physical and Theoretical Chemistry, NAWI Graz, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria
∥Ivoclar Vivadent AG, 9494 Schaan, Liechtenstein
S
* Supporting Information
ABSTRACT: Dimethacrylates are known to have good
photoreactivity, but their radical polymerization usually leads
to irregular, highly cross-linked, and brittle polymer networks
with broad thermal polymer phase transitions. Here, the
synthesis of mono- and difunctional β-allyl sulfones is
described, and those substances are introduced as potent
addition−fragmentation chain transfer (AFCT) reagents
leading to dimethacrylate networks with tunable properties.
By controlling the content and functionality of said AFCT
reagents, it is possible to achieve more homogeneous networks
with a narrow glass transition and an adjustable glass transition
temperature (Tg), rubber modulus of elasticity (Er), and
network density. In contrast to dimethacrylate networks
containing monomethacrylates as reactive diluents, the network architecture of the β-allyl sulfone-based dimethacrylate
networks is more homogeneous and the tunability of thermal and mechanical properties is much more enhanced. The reactivity
and polymerization were investigated using laser flash photolysis, photo-DSC, and NMR, while DMTA and swellability tests were
performed to characterize the polymer.
formed networks remain inhomogeneous due to the uncon-
trolled radical chain growth mechanism.
INTRODUCTION
■
UV-photopolymerization of (meth)acrylates paves the way to
convenient and low-energy processability.1,2 Polymers derived
via photopolymerization are attractive for protective and
decorative coatings,3,4 biomedical research,5−8 and 3D-lithog-
raphy techniques.9−13 While enabling a number of favorable
properties such as rapid curing, low-energy processing, good
storage stability, and the absence of solvents, the attainable
material properties are limited. This is strongly due to the lack
of control over the radical polymerization. Conventional UV-
photopolymerization of cross-linking monomers can be
described as a radical chain growth polymerization, leading to
materials with uncontrolled and inhomogeneous network
architecture. As a result, this network architecture yields
materials with broad thermal phase transitions and brittle
networks. Nevertheless, some photopolymerized resins con-
taining a substantial amount of reactive diluent (monofunc-
tional monomer) and cross-linkers with flexible spacers (e.g.,
ethylene glycol-based) may form materials having a narrow Tg
and enable tunability of mechanical properties.14 However,
such materials may generate toxicity problems due to the
migration of unreacted monofunctional monomers. Also, the
For the synthesis of photopolymer networks with increased
homogeneity, a controlled radical polymerization is desirable.
Alternatively, a change from the radical chain growth
mechanism toward a step growth-like system is a possibility.
A number of methods for controlled free radical polymerization
have been described in the literature (e.g., reversible addition−
fragmentation chain transfer (RAFT)15 and atom-transfer
radical polymerization (ATRP)16). AFCT15 reagents also
have the potential to regulate radical polymerization reactions.
The AFCT chemistry proceeds similarly to the polyreaction of
thiol−ene/yne17−19 systems in a mixed chain growth/step
growth-like mechanism without having their drawbacks of poor
storage stability and bad odor. For photopolymerizable cross-
linking systems the AFCT approach seems to be the most
promising technique, since RAFT and ATRP systems tend to
show strong absorbance in the UV/vis light region.
Received: July 29, 2014
Revised: October 3, 2014
© XXXX American Chemical Society
A
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Figure 1. Simplified models of network architectures.
NMR spectra were recorded on a Bruker AC 200 at 200 MHz (50
MHz for 13C); chemical shifts are given in ppm and were referenced to
the solvent residual peak (CDCl3). Multiplicities are referred to as s
(singlet), d (doublet), t (triplet), q (quartet), and m (multiplet).
Coupling constants are given in Hz. Silica gel chromatography was
The AFCT mechanism shows great potential in pushing an
uncontrolled radical chain growth polymerization reaction
toward a step growth-like manner (Figure 1).15,16 By adding
AFCT reagents to a radical polymerizable cross-linking
monomer mixture, the uncontrolled radical chain growth
mechanism can be altered and would ultimately produce
materials with a more homogeneous network structure and,
consequently, improved mechanical properties. β-Allyl sulfones
are promising AFCT reagents, and basic polymerization studies
on such substances have already been performed.20,21 However,
hardly any AFCT reagents aside from β-allyl sulfides22 have
been mentioned for the synthesis of photopolymerized cross-
linked functional materials. Research on AFCT reagents thus
far was mainly focused on molecular weight control of linear
polymers (e.g., styrene20 and methyl methacrylate23), for the
synthesis of hyperbranched polymers24 or end-group function-
alization.20,25,26 In general, highly cross-linked polymers with
AFCT reagents potentially pave the way for applications such
as shape memory polymers14,27 and covalent adaptable
networks.28
In this work we show, how β-allyl sulfones (AFCT reagents)
regulate the architecture of photopolymerized dimethacrylate
networks and consequently generate more homogeneous
polymer networks. By optimizing the synthesis of β-allyl
sulfones,29 a mono- and difunctional AFCT reagent have
successfully been isolated. The reactivity and the general
mechanism of β-allyl sulfones in photopolymerizations were
investigated by means of laser flash photolysis and photo-DSC.
β-Allyl sulfone-based dimethacrylate networks have been
formed through photopolymerization and were compared to
a standard dimethacrylate network and dimethacrylate net-
works containing a reactive monomethacrylate diluent. The
tunability of properties such as polymerization time (deter-
mined via photo-DSC), Tg, Er (determined via DMTA),
network density, and swellability has been evaluated.
performed with a Buchi MPLC-system equipped with the control unit
̈
C-620, fraction collector C-660, and UV-photometer C-635.
Commercial grade reagents and solvents were used without further
purification.
Synthesis of p-Toluenesulfonyl Iodide (pTSI). All work steps
were performed under light protection by working in the yellow light
lab where wavelengths below 480 nm are filtered (adhesive foils of the
company IFOHA were used to cover windows and fluorescent lamps).
At first, iodine (9.14 g, 36 mmol) was dissolved in approximately 150
mL of ethanol and then added to a stirred solution of sodium p-
toluenesulfinate (6.41 g, 36 mmol, 0.1 M in water). After the addition
was completed, the reaction was stirred for another 15−30 min. Then
the yellow precipitate (pTSI) was filtered by suction filtration and
washed with water. Afterward, the precipitate was dissolved in a
minimal amount of toluene and dried over Na2SO4. The solution was
then filtered into the same amount of cold petrol ether and pTSI was
crystallized at −20 °C. The yellow solid was filtered, washed with cold
petrol ether, and dried under vacuum. pTSI was isolated in 60% yield
1
3
(6.09 g). H NMR (200 MHz, CDCl3, δ, ppm): 7.75 (d, J = 8.2 Hz,
2H; Ar−H), 7.34 (d, 3J = 8.2 Hz, 2H; Ar−H), 2.48 (s, 3H; Ar−CH3).
13C NMR (50 MHz, CDCl3, δ): 147.5 (C4), 146.3 (C4), 129.7 (C3),
125.4 (C3), 21.8 (C1). Analytical data were in accordance with
literature values.30
Synthesis of β-Allyl Sulfones. The first step of this synthesis was
prepared under light protection. Depending on the target compound 1
equiv (1.69 g, 6 mmol; for monofunctional β-allyl sulfone MAS) or 2
equiv (3.38 g, 12 mmol; for difunctional β-allyl sulfone DAS) of pTSI
was placed into separate 100 mL round-bottom flasks, and 50 mL of
CH2Cl2 was added to each. Then 1 equiv of DEGEMA (1.21 g; for
MAS) or 1 equiv of TTEGDMA (1.98 g; for DAS) was added to the
reaction solutions. At this point the reaction solutions were placed in
the light (regular light bulb), and both reactions were stirred until the
methacrylate double bonds disappeared in the 1H NMR spectra
(additional TLC monitoring). In both cases the reaction took
approximately 3 h. After completion of the reaction step, 50 mL of
ethyl acetate was added to the reaction solutions, and all following
steps were again performed under light protection. CH2Cl2 was
evaporated, and the ethyl acetate phase washed with a 5 wt % solution
of sodium dithionite (2 × 30 mL) and water (2 × 30 mL). The
collected aqueous phases were reextracted with 30 mL of ethyl acetate,
and the combined organic phases were dried over Na2SO4. The
solutions were filtered and placed into three-neck round-bottom flasks
with reflux condensers. The reactions were flushed with argon, and 5
equiv of freshly distilled triethylamine (3.04 g, 30 mmol) was added
slowly to both reactions. The reactions were refluxed, and the progress
EXPERIMENTAL SECTION
■
Materials and General Methods. The photoinitiator (PI) bis(4-
methoxybenzoyl)diethylgermanium (Ivocerin) and the monomers
1,10-decanediol dimethacrylate (D3MA) and urethane dimethacrylate
(UDMA, isomeric mixture; CAS: 72869-86-4) were provided by
Ivoclar Vivadent AG. Tetra(ethylene glycol) dimethacrylate (TTEGD-
MA) was purchased from UCB Chemicals. Di(ethylene glycol) ethyl
ether methacrylate (DEGEMA) and methyl methacrylate (MMA)
were purchased from Sigma-Aldrich. Bis(2,6-dimethoxybenzoyl)(2,4,4-
trimethylpentyl)phosphine oxide (CGI403) was provided by Ciba SC.
B
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Scheme 1. Synthesis of β-Allyl Sulfones (AS)
Figure 2. Monomers, AFCT reagents, and PI used for network studies: 1,10-decanediol dimethacrylate (D3MA), urethane dimethacrylate (UDMA),
di(ethylene glycol) ethyl ether methacrylate (DEGEMA), mono-β-allyl sulfone (MAS), di-β-allyl sulfone (DAS), and bis(4-methoxybenzoyl)-
diethylgermanium (Ivocerin).
of the reactions was tracked via 1H NMR analysis. After completion of
the reactions, the solutions were cooled and then washed with 1 N
HCl (2 × 50 mL) and water (50 mL). The aqueous phases were
reextracted with ethyl acetate and the combined organic phases dried
over Na2SO4. The solvent was evaporated, and the crude products
were purified via silica column chromatography (PE/EE: 1/3).
2-(2-Ethoxyethoxy)ethyl 2-(tosylmethyl)acrylate (MAS): 56% yield
adjusted to achieve an absorbance of ca. 0.3 at 355 nm. The transient
absorption spectra were recorded in a quartz cuvette (1 cm × 1 cm).
Samples were purged and constantly bubbled with argon to refresh the
sample and avoid sample decomposition. The rate constants for the
addition of the phosphinoyl radicals to the monomer double bonds,
kadd, were determined in pseudo-first-order according to the equation
kobs = k0 + kadd quencher. Furthermore, the global analysis software Pro-K
c
1
3
(Applied Photophysics) was employed.
(1.20 g); H NMR (200 MHz, CDCl3, δ, ppm): 7.73 (d, J = 8.2 Hz,
3
Photo-DSC. A Netzsch DSC 204 F1 with autosampler was used to
perform the photo-DSC measurements. All measurements were
conducted isocratic at 25 °C under a N2 atmosphere. 10 1 mg of
a monomer formulation was weighed in accurately into an aluminum
DSC pan, which was then placed in the DSC chamber. After the
sample chamber had been purged with N2 (N2 flow = 20 mL min−1)
for 4 min, the samples were irradiated with filtered UV-light (400−500
nm) from an Exfo OmniCure series 2000 for a defined duration at an
intensity of 1 W cm−2 at the exit of the light guide (corresponds to
∼20 mW cm−2 on the surface of the sample). The heat flow of the
polymerization reaction was recorded as a function of time.
Dynamic Mechanical Thermal Analysis (DMTA). DMTA
measurements were performed with an Anton Paar MCR 301 with a
CTD 450 oven and a SRF 12 measuring system. Polymer specimens
(∼5 × 2 × 40 mm3) were tested in torsion mode with a frequency of 1
Hz and strain of 0.1%. The temperature was increased from −100 to
200 °C with a heating rate of 2 °C min−1. The storage modulus and
the loss factor of the polymer samples were recorded with the software
Rheoplus/32 V3.40 from Anton Paar.
2H; Ar−H), 7.32 (d, J = 8.2 Hz, 2H; Ar−H), 6.52 (s, 1H; =CH2),
5.89 (s, 1H; =CH2), 4.12 (m, 4H; SO2−CH2−, OOC−CH2−), 3.62
(m, 6H; −CH2−O−CH2−CH2−), 3.53 (q, 3J = 7.0 Hz, 2H; O−CH2−
3
CH3), 2.43 (s, 3H; Ar−CH3), 1.21 (t, J = 7.0 Hz, 3H; O−CH2−
CH3); 13C NMR (50 MHz, CDCl3, δ, ppm): 164.8 (CO), 144.9
(C4), 135.4 (C4), 133.6 (C2), 129.7 (C3), 128.9 (C4), 128.8 (C3),
70.7 (C2), 69.8 (C2), 68.8 (C2), 66.7 (C2), 64.5 (C2), 57.5 (C2),
21.6 (C1), 15.1 (C1); Anal. Calcd for C17H24O6S: C 57.28, H 6.79, S
9.00. Found: C 56.51, H 6.71, S 8.72.
((Oxybis(ethane-2,1-diyl))bis(oxy))bis(ethane-2,1-diyl) bis(2-
(tosylmethyl)acrylate) (DAS): 70% yield (2.68 g); 1H NMR (200
MHz, CDCl3, δ, ppm): 7.73 (d, 3J = 8.2 Hz, 4H; Ar−H), 7.32 (d, 3J =
8.2 Hz, 4H; Ar−H), 6.52 (s, 2H; =CH2), 5.89 (s, 2H; =CH2), 4.14 (m,
8H; OOC−CH2−, SO2−CH2−), 3.62 (m, 12H; −CH2−O−CH2−
CH2−), 2.43 (s, 6H; Ar−CH3); 13C NMR (50 MHz, CDCl3, δ, ppm):
164.9 (CO), 144.9 (C4), 135.4 (C4), 133.6 (C2), 129.7 (C3), 128.9
(C4), 128.8 (C3), 70.7 (C2), 68.8 (C2), 64.5 (C2), 57.5 (C2), 21.6
(C1); Anal. Calcd for C30H38O11S2: C 56.41, H 6.00, S 10.04. Found:
C 56.36, H 5.89, S 9.84.
Synthesis of Polymer Networks for Testing. All prepared
monomer formulations were mixed with 0.97 mol % PI (Ivocerin) in
an ultrasonic bath for 30 min at ambient temperature. The
formulations with β-allyl sulfones added showed good storage stability
over the course of 90 days. Rheological data have been provided in the
Supporting Information (Table S1). For photo-DSC measurements,
the monomer formulations were used. For DMTA analysis and
swellability experiments, polymer specimens were fabricated by
pouring the monomer formulations into a silicone mold (sticks, 5 ×
2 × 40 mm3 for DMTA; disks, d = 4 mm, h = 2 mm for swellability
tests) and then curing the samples for 20 min in a Lumamat 100 light
oven (provided by Ivoclar Vivadent AG) with six Osram Dulux L Blue
18 W lamps. The emitted wavelength spectrum was 400−580 nm at a
measured total intensity of ∼20 mW cm−2 determined with an Ocean
Optics USB 200+ spectrometer.
Swellability Tests. The polymer disks (3 disks per sample) were
submerged in ethanol and stored at ambient temperature for 7 days.
200 ppm of hydroquinone monomethyl ether was added to the
ethanol to prevent free-radical reactions. The ethanol was replaced two
times, after 1 and 4 days. The polymer disks were dried using a paper
towel and then weighed. Afterward, the disks were placed in a 60 °C
vacuum oven and dried until a constant weight was reached.
RESULTS AND DISCUSSION
■
Synthesis. Prior to investigating the novel networks, the β-
allyl sulfone precursors had to be synthesized (Scheme 1). The
synthesis was performed in three reaction steps, and due to the
light sensitivity of the intermediate iodine compounds, it was
carried out in a light-protected laboratory (light with
wavelengths below 480 nm was filtered). The reagent pTSI
was prepared in a simple reaction of sodium p-toluenesulfinate
and iodine in an ethanol/water mixture.30,31
Laser Flash Photolysis. Nanosecond transient absorption experi-
ments were performed on a LKS80 spectrometer (Applied Photo-
physics, UK). Excitation was performed using the third harmonic (355
nm, 10−20 mJ/pulse, ca. 8 ns) of a Spitlight Compact 100 (InnoLas,
Germany) Nd:YAG laser. Concentration of CGI403 in acetonitrile was
For the first reaction step, depending on the desired product,
1 equiv (for MAS) or 2 equiv (for DAS) of pTSI was added to
C
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Scheme 2. Photopolymerization Mechanism of a β-Allyl Sulfone-Based Methacrylate System
a solution of dry CH2Cl2 and DEGEMA (1 equiv for MAS) or
TTEGDMA (1 equiv for DAS).32,33 The reactions were
exposed to visible light and monitored via 1H NMR
spectroscopy by looking at the decrease of double bond signal
(δ = 6.08 and 5.53 ppm) and the formation of the iodine
2
compound I (δ = 4.47 and 3.90 ppm; d; J = 13.8 Hz).
Triethylamine (Et3N) was added to deprotonate the iodine
compound and consequently cleave off the iodine to yield the
kinetic product K. The reaction mixture was refluxed overnight
to form the desired thermodynamically stable β-allyl sulfones
(AS). Purification of both products was achieved by silica gel
column chromatography. MAS and DAS were isolated in a
satisfactory yield of 56% and 70%, respectively (Figure 2).
Reactivity of β-Allyl Sulfones. To explore the reactivity of
the β-allyl sulfones in photopolymerizations, laser flash
photolysis (LFP) was employed to determine the addition
rate constant of initiator radicals to the double bond. For these
experiments, the monofunctional MAS as well as the
Figure 3. Pseudo-first-order decay rate constant (kobs) of the
phosphinoyl radical P• versus monomer concentration; LFP measured
in acetonitrile at ambient temperature.
monomers DEGEMA and MMA were used. Furthermore, it
was attempted to determine the follow-up kinetics of the β-
scission of intermediate radical B (Scheme 2). For these
experiments the initiator Ivocerin was not employed because of
two significant disadvantages: (1) the very fast addition
kinetics34 near the limit of the experimental setup and (2)
exhibiting transient absorption below 360 nm. The latter being
problematic because sulfonyl radicals exhibit a λmax at ∼330
nm35 (Figure S1). Therefore, a bisacylphosphine oxide PI
(CGI403) was used. The phosphinoyl radical from this initiator
only exhibits weak absorption at lower wavelengths because of
the nonaromatic substituent at the central phosphorus.36
After photoexcitation at 355 nm the addition rate constant of
the phosphinoyl radical P• (Figure S2) to monomer and AFCT
reagent was measured by following the decay of the absorption
at 450 nm. Figure 3 shows the dependence of the observed
(pseudo-first-order) rate constant on the concentration of
reagent.
be seen that the reactivity of the MAS is in a similar range as
the methacrylate, but importantly slightly slower.
As is clear from the transient spectra, recorded at different
delays after irradiation (Figure 4), there is a second species
visible in the spectrum with a λmax at ∼315 nm. This peak is
attributed to radical B (Scheme 2) resulting from the addition
of P• to MAS. This is further evident when comparing the two
decays at 345 and 450 nm. To extract the kinetics for the decay
of the second species, global analysis of the complete spectrum
was performed. This was necessary as the phosphinoyl radical
shows some absorption overlapping with the target radical and
the expected sulfonyl radical exhibiting a similar absorption
spectrum.
The resulting rate constant for the decay of the second
species is (5 1) × 104 s−1. This is in the comparable range for
sulfonyl β-scission of a tertiary radical next to an ester group,
which was determined to be 2 × 104 s−1.37 However, it has to
be noted though that this rate constant exhibits a dependence
on concentration of MAS. This is probably caused by the
The addition rate constants (kadd) are (2.84 0.04) × 107
and (3.84
0.08) × 107 M−1 s−1 for MAS and DEGEMA,
respectively. This is also comparable to the addition rate
constant of MMA (kadd = (3.99 0.06) × 107 M−1 s−1). It can
D
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Figure 4. (a) Kinetics of the transient absorptions after laser flash and (b) transient absorption spectra at different delays, of 0.3 mM CGI403 and 25
mM MAS in acetonitrile.
reaction of B with the formed sulfonyl radicals, recombination
having to occur because there is no other path available.
Additionally, hydrogen abstraction from other MAS molecules
will lead to an increase in the observed decay rate of the radical
B.
When comparing the determined rate constants to the typical
rates of polymerization of methacrylates (<103 M−1 s−1),38 β-
scission is expected to be the major pathway of reaction without
significant contributions of homopolymerization or addition
reactions before cleavage occurs. Moreover, sulfonyl radicals are
very reactive toward methacrylates especially (∼1 × 109 M−1
s−1),39 leading to fast reinitiation of a new radical chain.
Photopolymerization Mechanism of β-Allyl Sulfones
with Methacrylates. In Scheme 2, the mechanism for a mixed
chain growth/step growth-like β-allyl sulfone-based methacry-
late network formation is shown, similar to how it is proposed
for various AFCT reagents in the literature.15,16,24
according to the literature,40 SR has a negligible effect on
molecular weight regulation. Furthermore, the resulting tertiary
radical F has low reactivity and could have a negative effect on
the reaction rate.
To elucidate the proposed mechanistic steps and provide
estimation for the ratios of MA vs CT during the β-allyl
sulfone/dimethacrylate network formation, polymerization
experiments of the monofunctional AFCT reagent MAS (20
DB, DB = double-bond equivalents) in the monofunctional
DEGEMA (Figure 5) have been performed.
After the formation of initiating radicals (IR•), which are
generated by exciting a PI (germanium-based Ivocerin in this
case), the radicals can potentially attack either a methacrylate
double bond (methacrylate addition, MA) or the double bond
of a β-allyl sulfone (chain transfer, CT). The ensuing steps are
dependent upon which of the two events takes place. After an
MA step, the new radical A can either again undergo a standard
radical chain growth step (MA) or participate in a CT step.
However, by attacking an AFCT reagent, the CT step is
consequently guiding the polymerization mechanism in a step
growth-like manner. In the case of a CT step, the growing
radical chain will be terminated by forming an intermediate
radical B that undergoes fragmentation and forms a sulfonyl
radical C and a new double bond (compound D). This sulfonyl
radical has again two options. When undergoing a CT step,
which results in an event that has the same products as starting
materials, this event will continue in a “sulfonyl radical
exchange mechanism”, until a methacrylate double bond is
attacked by the sulfonyl radical. The MA step leads to a
propagating radical E that also has the option to perform either
an MA or a CT step. It has to be noted that one subsequent
reaction (SR) can take place during this polymerization event.
After a β-allyl sulfone has reacted, a new, sterically hindered
methacrylate-like double bond remains (compound D). During
the network formation this double bond can potentially be
attacked by any existing radical, for example a radical of a
Figure 5. 1H NMR of 20 DB MAS in DEGEMA after photo-
polymerization with 3 mol % Ivocerin (5 min, 1 W cm−2, 400−500
nm).
When MAS (20 DB) is mixed with DEGEMA and 3 mol %
of PI Ivocerin and then irradiated by UV-light (400−500 nm, 1
W cm−2), the resulting polymer can be analyzed based on the
photo-DSC plots and by 1H NMR experiments. As a result, the
conversion of the different double bonds (MA, CT, and SR)
could be determined. Figure 5 represents a polymer sample of a
DEGEMA/MAS 20 DB formulation after irradiation of 5 min
with the photo-DSC. The double bond hydrogens of the
monomer DEGEMA (H1 and H2) and the AFCT reagent MAS
•
growing polymer chain Mn , leading to an additional MA step
which increases cross-linking in the network. However,
E
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(H3 and H4) are displayed in the 1H NMR spectrum as well as
the hydrogens of the newly formed double bonds (H5−8). The
decrease of the H1 and H2 integrals gives the yield of the MA
step, and the CT step is quantified by the decrease of the H3
and H4 integrals. The conversion of the newly formed double
bonds (SR step) was determined by monitoring the decrease of
the H5−8 integrals. The 1H NMR integrals for the double bond
signals of DEGEMA (H1−2), MAS (H3−4), and the newly
formed double bonds (H5−8) are evaluated after set time
intervals of irradiation and correlated to the start integral of the
respective double bond signals to yield the double bond
conversions (DBCs). As reference, the signals of the aromatic
hydrogens originating from MAS are used. The reference
integral, for calculating DBC of the SR step, was always set to
be equivalent to the decrease of the double bond signals for
MAS, with the assumption that for every CT step a new double
bond is formed (β-scission is the major pathway).
reaction step (MA, CT, and SR) needed to be evaluated from
photo-DSC experiments with the help of 1H NMR
spectroscopic measurements. Photo-DSC plots (600 s) of
pure DEGEMA and a DEGEMA/MAS 20 DB formulation
were analyzed (Figure S3). By integrating the photo-DSC plots
of the DEGEMA homopolymerization (ΔHDEGEMA) and
looking at the corresponding 1H NMR spectra of the polymers
(DBCNMR), the theoretical heat of polymerization for
DEGEMA (ΔH0,DEGEMA) could be calculated (eq 1) as 60 kJ
mol−1, which is in good agreement with literature (62 kJ
mol−1).41
ΔHDEGEMA
DBCNMR
ΔH0,DEGEMA
=
(1)
ΔHDEGEMA
DBCphoto‐DSC
=
mDEGEMA
ΔH0,DEGEMA
mtot
(2)
Multiple experiments have been performed with different
irradiation periods, and in Figure 6 and Table S2 the evolution
Table 1. Experimental DBC Values Determined via Photo-
DSC and NMR
ΔHDEGEMA
DBCphoto‑DSC
(%)
DBCNMR
(%)
formulation
DEGEMA
DEGEMA/MAS (20DB)
(J g−1
)
282
191
95
90
93
The DBC for DEGEMA in a DEGEMA/MAS 20 DB
formulation was determined by NMR spectroscopy to be
DBCNMR = 90%. Given this value, the theoretical heat coming
from the MA step can be calculated to be 185 J g−1, which is
very close to the measured value of 191 J g−1. This means that
the excess heat (6 J g−1), which amounts to only 3% of the total
heat, can be attributed to the CT and SR steps. With this
calculation the assumptions were made that the CT step is
energy neutral and that the developed heat of the side reactions
can be neglected.40 Taking into account that only 80 mol % in a
DEGEMA/MAS 20 DB formulation are methacrylate double
■
▲
Figure 6. Graphical evolution of DBC for MA ( ), CT ( ), and SR
( ) versus irradiation time.
⧫
of the DBCs is illustrated. It shows how methacrylate and β-
allyl sulfone react very efficiently with each other and are
consumed in a similar rate over the whole irradiation period of
ti = 600 s which might enable the formation of a homogeneous
network. The experiment shows that the MA reaction is slightly
preferred. Nevertheless, it was shown that β-allyl sulfones are
very potent AFCT reagents (DBC > 80%) and exhibit good
reactivity in a radical copolymerization reaction with meth-
acrylates. It needs to be stated that the conducted experiment
hints to the proposed photopolymerization mechanism and
reflects the ratios of MA to CT in cross-linking dimethacrylate
networks. However, the mobility of the polymer chains
decreases significantly during a dimethacrylate photopolymeri-
zation, where the gel point is rapidly reached, and this can lead
to a significant change in kinetics and the ratio of MA to CT
and SR, respectively. Moreover, by photopolymerizing the
samples up to a higher conversion, the kinetics and the ratio of
bonds, the theoretical heat of polymerization (ΔH0,DEGEMA
)
needs to be corrected by the weight fraction of DEGEMA
(mass of DEGEMA mDEGEMA divided by the total mass of the
formulation mtot). With eq 2 the DBC of DEGEMA in a
DEGEMA/MAS formulation could be calculated via photo-
DSC (Table 1, DBCphoto-DSC) and is 93%. The correlation
between DBCphoto‑DSC and DBCNMR is satisfactory, and the
slightly larger value of DBCphoto‑DSC can be attributed to side
reactions such as SR, which lead to a slightly larger value for
ΔHDEGEMA in the calculation. Consequently, this approximation
can be used for cross-linking systems to indicate the DBC
1
where H NMR spectroscopic analysis is not an option.
Photoreactivity of β-Allyl Sulfone-Based Dimethacry-
late Formulations. β-Allyl sulfone-based dimethacrylate
formulations potentially delay a photopolymerization reaction
by driving the polymerization process toward a mixed chain
growth/step growth-like manner. The dimension of this delay
and the potential increase in DBC through gel point delay
should be investigated. Therefore, the photoreactivity of
dimethacrylate formulations with AFCT reagents in various
amounts and functionality was investigated by the means of
photo-DSC (Table 2 and Figure S4). A pure dimethacrylate
formulation of UDMA and D3MA (1/1 molar ratio, 2M) was
examined as reference resin for network A. This reference
formulation was compared with the formulations for networks
1
MA to CT can change significantly. Through the H NMR
experiments only the overall conversions of each double bond
for this specific system can be determined.
In the following chapters the focus will shift to β-allyl
sulfone-based dimethacrylate networks, which cannot be
analyzed via 1H NMR spectroscopy due to insolubility.
Therefore, the DBC of the tested monofunctional formulation
(DEGEMA/MAS 20 DB) should be calculated by means of
photo-DSC. The unknown heat of polymerization of each
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ΔH0,2M is calculated using molecular weight (Mw,D3MA = 310.43
g mol−1, Mw,UDMA = 470.56 g mol−1) and theoretical enthalpy
(ΔH0,D3MA = 120 kJ mol−1, ΔH0,UDMA = 100 kJ mol−1)42,43 of
D3MA and UDMA. In theory, the reaction heat of the CT step
should be close to 0, and following this assumption the DBC of
the experiments with AFCT reagent added can be determined
by correction of ΔH0,2M according the weight percent (wt %) of
2M in each formulation (m2M = mass of 2M; mtot = total mass
of the formulation; eq 3). The reference formulation, solely
based on dimethacrylates, forms the network A with a DBC of
74%. The addition of mono- (MAS) or difunctional β-allyl
sulfones (DAS) results in a higher DBC of up to 86% with a
content of 25 DB MAS and DAS, respectively. Pure
dimethacrylate-based photopolymers sometimes suffer from
low DBC, which is due to the early gelation and the
corresponding decreased mobility of the growing polymer
chains. From these results, it can be seen that AFCT reagents
increase the DBC of methacrylates (Table 2).
Table 2. Photo-DSC Results of Polymerized Formulations
a
network
composition
ΔH (J g−1
)
DBC (%) t95% (s)
A
B
C
D
E
2M
223
179
173
160
187
181
167
74
82
84
86
83
85
86
70
105
114
128
99
2M/MAS (16.67 DB)
2M/MAS (20 DB)
2M/MAS (25 DB)
2M/DAS (16.67 DB)
2M/DAS (20 DB)
2M/DAS (25 DB)
F
104
109
G
a
t95% = time until 95% of reaction heat has been developed.
B-G (2M with 16.67, 20, and 25 DB of monofunctional transfer
agent MAS and the difunctional DAS, respectively). A mixture
of 2M with 25 DB MAS for instance indicates that 25% of all
double bonds in the formulation are double bonds from the
respective AFCT compound (MAS), and the remaining double
bonds in the formulation are methacrylate double bonds from
2M. For all samples 0.97 mol % Ivocerin were used as PI and
the photopolymerization experiments were carried out under a
N2 atmosphere in an isothermal mode at 25 °C. The samples
were irradiated for 5 min each (1 W cm−2 at the tip of the light
guide, 400−500 nm), and the resulting DSC plots were
analyzed with respect to their exothermic heat evolution and
reaction time. The heat of polymerization (ΔH) for the
dimethacrylate homopolymerization of network A is higher
compared to the heat of polymerization with AFCT reagents
added. For the network A based on 2M the DBC can be
determined by dividing the measured ΔH through the
theoretical heat of polymerization (ΔH0,2M = 299.54 J g−1).
ΔH2M
ΔH0,2M
DBC2M
=
m2M
mtot
(3)
The time where 95% (t95%) of the reaction heat has been
developed can be seen as a measure for the rate and progress of
the photopolymerization. Overall, the reference monomer
mixture A (only dimethacrylates) shows the fastest polymer-
ization with a t95% of 70 s. The β-allyl sulfone-based
dimethacrylate resins exhibit less photoreactivity in contrast
to the reference A. The overall reaction takes longer by a factor
of max 1.8 (formulations D vs A), which is tolerable
Figure 7. Storage modulus (left) and loss factor curves (right) of different networks.
G
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considering the much more substantial retardation effect of
other potential chain transfer reagents such as, e.g., RAFT
reagents.44
polymer network does not have enhanced homogeneity
compared to the new β-allyl sulfone-based polymer networks.
Swellability and Network Density. The swellability and
gel fraction of a polymer network correspond to its network
density.45,46 By swelling polymer disks of the tested networks
A−J (3 disks per network), the swellability (S) and gel fraction
(G) were determined (eqs 4 and 5).
Glass Transition Temperature (Tg) and Rubber
Modulus of Elasticity (Er). It was proposed that by adding
β-allyl sulfone to a dimethacrylate formulation the polymer
network architecture can be altered leading to more defined
and tunable thermal transitions. In order to test the thermal
transitions of the synthesized materials, they were characterized
by dynamic mechanical thermal analysis (DMTA). An
additional standard UV-photopolymerized methacrylate net-
work with 25 DB monomethacrylate DEGEMA (network J)
was also analyzed supplementary to network A as reference.
Usually, monofunctional methacrylates are used as reactive
diluents to tune the viscosity of the photopolymerizable resins.
Additionally, the mechanical and thermal properties such as the
Tg or Er can be influenced. Figure 7 shows the resulting curves
of the DMTA measurements.
mswollen
mdry
S =
(4)
mdry
G =
mstart
(5)
The disks were weighed at the beginning (mstart) and then
submerged in ethanol for 7 days. After the swelling period the
polymer samples were weighed in the swollen state (mswollen
)
and then dried in a vacuum oven to receive the dry mass of the
disks (mdry). This experiment together with the Tg of the
photopolymers gives information about the network density of
the polymer networks and helps to understand the concept of
β-allyl sulfone/dimethacrylate networks. As reference networks,
the 2M-based dimethacrylate network A and networks H−J
with added monofunctional methacrylate DEGEMA are
analyzed. The standard deviations for swellability values and
for the gel fraction were <0.8% and <0.4%, respectively, for all
measurements.
In Figure 7a (storage modulus and loss factor plots), the
novel β-allyl sulfone-based dimethacrylate networks with
monofunctional AFCT reagent MAS (16.67, 20, and 25 DB;
B−D) are compared to the reference network A. As can be seen
in Table 3, the thermal glass transition of the networks B−D
Table 3. DMTA Results of Measured Networks
network
composition
Tg (°C) fwhm (°C) Er (MPa)
A
B
C
D
E
F
G
J
2M
148
65
76
7
Table 4. Gel Fraction and Swellability of Polymer Networks
Correlated with Tg
2M/MAS (16.67 DB)
2M/MAS (20 DB)
2M/MAS (25 DB)
2M/DAS (16.67 DB)
2M/DAS (20 DB)
2M/DAS (25 DB)
2M/DEGEMA (25 DB)
30
25
25
33
29
26
56
53
5
39
3
swellability gel fraction
Tg
(°C)
83
13
10
7
network
composition
(wt %)
(wt %)
73
A
B
C
D
E
F
G
H
I
2M
3.6
17.0
18.7
22.4
5.9
98.6
98.1
97.0
93.4
98.8
97.9
97.5
98.9
98.9
98.5
148
65
53
39
83
73
63
63
2M/MAS (16.67 DB)
2M/MAS (20 DB)
106
36
2M/MAS (25 DB)
2M/DAS (16.67 DB)
2M/DAS (20 DB)
9.0
shows a considerably lower Tg (65−39 °C) and the width of
the peak (full width at half-maximum values, fwhm) decreases
with increasing AFCT reagent content (30−25 °C). The Er of
A (at T > Tg; minimum of storage modulus, 76 MPa) is
significantly higher compared to the rubber moduli of B−D
with 7, 5, and 3 MPa. This is mainly due to the higher cross-
linked, but yet less homogeneous network. Consequently, in
the case of β-allyl sulfone/dimethacrylate networks the Tg can
be tuned by the increase of AFCT content, while also achieving
a narrow glass transition. Importantly, the modulus below glass
transition is not changed significantly. In Figure 7b (storage
modulus and loss factor plots) the β-allyl sulfone-based
networks are compared to the standard dimethacrylate network
A and network J containing reactive diluent DEGEMA instead
of an AFCT reagent. The networks based on monofunctional
MAS exhibit a lower Tg (65−39 °C) and fwhm (30−25 °C)
compared to the difunctional DAS-based networks (Tg = 83−
63 °C; fwhm = 33−26 °C), although the same amount of active
AFCT groups is present in the formulations. This can be
rationalized as DAS has the potential to act as a cross-linker,
whereas MAS can be considered a reactive diluent. Reference
network J does not have a sharp thermal transition (fwhm = 56
°C), which shows that monofunctional methacrylates as
diluents do not enable easy tuning of the network properties
comparably to the β-allyl sulfones. The sharpness of the glass
transition cannot be adequately tuned because the resulting
2M/DAS (25 DB)
11.4
6.3
2M/DEGEMA (16.67 DB)
2M/DEGEMA (20 DB)
2M/DEGEMA (25 DB)
7.6
J
9.8
106
Generally, the gel fraction (98.1−93.4%) of the synthesized
β-allyl sulfone/dimethacrylate networks is slightly less than for
the reference networks A and H−J (98.9−98.5%). Network D
with the highest content of monofunctional AFCT reagent
shows the lowest gel fraction (93.4%), which can be explained
by the migration of unreacted MAS and the less cross-linked
network with the lowest Tg influencing the network mobility
already at the experiment temperature. Networks B, C, and E−
G still have a high gel fraction (>97%) and a lower and more
homogeneous network density compared to network A, which
can be concluded by the increase in swellability of the networks
B−G. Although pure methacrylate networks with monofunc-
tional DEGEMA as reactive diluents (H−J) have also increased
swellability and exhibit comparable gel fraction as dimethacry-
late networks with AFCT content, the Tg is much higher and
the glass transition broader as can be seen with network J. This
shows that with the addition of β-allyl sulfones as AFCT
reagents to dimethacrylate formulations a new tool for tuning
the network properties and architecture of highly cross-linked
photopolymer networks could be established.
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The authors thank TU Graz/NAWI Graz for the use of the
LFP equipment. The authors also thank TU Wien for support
in the framework of Lion 2+.
CONCLUSIONS
■
We have introduced a new concept to control the polymer
network formation in the radical photopolymerization of
dimethacrylates. A mono- and difunctional β-allyl sulfone
were successfully synthesized. The AFCT mechanism and
reactivity were investigated by LFP, employing the monofunc-
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ASSOCIATED CONTENT
* Supporting Information
LFP spectrum of tosyl radical; photo-DSC plots for DEGEMA
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